1. Field of the Invention
The present invention relates to a crystal growth method and a semiconductor device, and more particularly to a crystal growth method for a nitride-based compound semiconductor and a semiconductor device formed thereby.
2. Description of Related Art
GaN-based compound semiconductors such as GaN, InGaN, and AlGaN are receiving attention as materials for blue light-emitting diodes (LEDs) and laser diodes (LDs). For their good thermal and environmental resistance, GaN-based compound semiconductors are today applied in and developed for electronic devices as well.
On the other hand, with GaN-based compound semiconductors, bulk crystal growth is difficult, and this makes practically usable GaN substrates extremely expensive. For this reason, most substrates for growth of nitride semiconductors currently in wide practical use are those of sapphire, and the commonly used method is to grow GaN epitaxially on top of a single crystalline sapphire substrate by a metal organic vapor phase epitaxy (MOVPE) method or the like.
Here, a sapphire substrate has a different lattice constant from GaN, and therefore according to a conventionally proposed method, first, a buffer layer of MN or GaN (low-temperature-grown buffer layer) is grown on top of a sapphire substrate at low temperature so as to alleviate lattice distortion in this low-temperature-grown buffer layer, and thereafter a GaN layer is grown on top. This method is disclosed, for example, in JP-A-S63-188938. The introduction of this low-temperature-grown buffer layer made single crystalline epitaxial growth of GaN possible, and led to the practical use of light-emitting diodes (LEDs) covering an ultraviolet to green region.
Inconveniently, however, even that method is ineffective in coping with lattice mismatch between substrate and crystal, with the result that the grown GaN has numberless defects. These defects will probably prove a hindrance in the fabrication of GaN-based LDs (laser diodes).
On the other hand, recent years have seen reports of, as methods that help reduce the density of defects ascribable to the difference in lattice constant between sapphire and GaN, crystal growth technologies such as ELO (epitaxial lateral overgrowth), FIELO (facet-initiated epitaxial lateral overgrowth), and pendeo-epitaxy, and these technologies now yield GaN epitaxial wafers with excellent high crystallinity. Methods such as ELO and FIELO produce single crystalline GaN layers with low defect density, and permit an LED structure to be formed on top of such a GaN layer or on top of a GaN layer of a sapphire substrate using a low-temperature-grown buffer layer. A crystal growth technology using ELO is disclosed, for example, in Appl. 71 (18) 2638 (1997), and a crystal growth technology using FIELO is disclosed, for example, in Japan. J. Appl. Phys. 38, L184 (1999). A crystal growth technology using pendeo-epitaxy is disclosed, for example, in MRS Internet J. Nitride Semicond. Res. 4S1, G3.38 (1999).
Furthermore, with the aim of reducing defect density and obtaining high-quality epitaxial layers, according to another conventionally proposed method, an anti-surfactant region (residual Si portion) is formed on a substrate, and a GaN-based semiconductor is grown while a void is formed over that region. This growth method is disclosed, for example, in JP-A-2000-277435.
On the other hand, recent years have seen high expectations for ultraviolet light-emitting devices using nitride semiconductors as new light sources for use in sterilization and water purification, various fields of medicine, quick decomposition of polluting substances, etc.
Inconveniently, however, with the conventional methods mentioned above, in high-Al-content AlGaN ternary alloy growth which is required for the formation of ultraviolet light-emitting devices, the large differences in lattice constant and in thermal expansion coefficient between substrate and epitaxial layer tend to cause development of cracks or a warp in the substrate.
Cracks disrupt the device structure, leading to unsatisfactory characteristics, and reduces the number of acceptable devices obtained from a single wafer, leading to diminished yields. On the other hand, a warp in the substrate not only makes the substrate prone to break during handling, but also makes it difficult, when forming a mask pattern on the substrate in a photolithography step or the like in the device process, to achieve uniform focus across the substrate surface, leading to diminished yields in device fabrication.
Against the background discussed above, in the application of nitride-based compound semiconductors, in particular in their application in the field of light-emitting devices such as LEDs where they are expected to offer wider emission wavelength ranges, there is demand for reduction of cracks and suppression of a warp in the substrate. In particular, in epitaxial growth of high-Al-content eutectic crystals which are required for light emission in an ultraviolet region, there is pressing demand for reduction of dislocations and cracks and suppression of a warp in the substrate.